Tufa

In the late Republican period (starting ca. 200 B.C.), slabs of tufa (a soft, volcanic stone easily found in central Italy) were often chosen to face a core construction of concrete as in the circular temple (Temple "C") in Largo Argentina in Rome. Or, irregular shaped, fist-sized tufa blocks created a wall mosaic covering the concrete (opus incertum) (B48). During the last century of the Republic and into the early empire, the favored method was to face concrete construction by regularized, diagonally arranged, square blocks of tufa (opus reticulatum), as we see in these examples from Hadrian's Villa at Tivoli (B50, B51). These small, square blocks were shaped like cones, their pointed ends firmly embedded in concrete.